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Temperatures in the western United States are set to soar to 120 degrees in a “rare, dangerous and deadly” heatwave, forecasters have warned.

The warnings have triggered fears of potentially devastating wildfires and power cuts that will knock out air-conditioning systems, leading to dangers of heat-related fatalities, in a vast swathe of territory stretching from Oklahoma to California - or an area inhabited by 66 million people, one-fifth of the US population.

They came as a three-year-old boy died in sweltering temperatures in Houston, Texas after getting locked inside his family’s car.

The boy was believed to have become trapped by the security locking system after going inside to retrieve a toy. He died after going into cardiac arrest, reports said.

Around 800 fire-fighters were dropping vast quantities of fire retardant from aircraft onto burning scrubland in an area of steep and rugged terrain that is difficult to reach on foot, reports said.

Two other brush fires were reported to be raging in New Mexico and another in Arizona.

The most intense heat was expected to be felt in southern California’s Coachella Valley, where temperatures were predicted to reach 120 degrees. The rising temperatures were expected to start from the weekend.

Phoenix, Arizona was preparing for temperatures of between 115 and 120 degrees while Las Vegas in Nevada was expected to experience a high of around 118.